1. Field of the Invention
The present invention relates to a multipoint grouting method for effecting multipoint-injection of a ground improving material into ground to be improved such as weak ground, soft ground, flimsy ground, loose ground or the like, and an apparatus therefor. In particular, it relates to a multipoint grouting method and an apparatus therefor which are not only capable of, with respect to ground having soil layers different from each other in soil condition, concurrently or selectively applying optimum injection to each of the soil layers but also capable of one-, two- or three-dimensionally injecting a ground improving material into ground, and which are further capable of flexibly controlling injection from a plurality of injection pipes and capable of carrying out injection concurrently from the plurality of the injection pipes to thereby increase reliability of permeation grouting into a very fine-grained soil layer and to thereby enable a shortened execution period to be realized by rapid execution.
The term “ground improving material” used herein means a ground solidifying injection material for strengthening ground to be improved such as weak ground, soft ground, flimsy ground or the like or for solidifying such ground to cut off water, an injection material (grouting material) for solidifying an environmental pollutant such as industrial wastes, a solidifying material for forming a cut-off layer to prevent leakage of a deleterious substance from an environmental pollutant, an injection material (grouting material) containing a chemical substance for rendering an environmental pollutant nonpolluting, a heavy metal immobilizing material for chemically inactivating a heavy metal, or the like.
2. Description of the Prior Art
In general, ground has soil layers which are different from each other in coefficient of water permeability, porosity or the like, and thus the soil layers are different from each other in soil condition such as soil texture. In injection of a grouting material (injection material) into ground of such a type, heretofore, a single injection pipe has been inserted or a plurality of injection pipes have been inserted at intervals into the ground, and the grouting material has been sequentially injected into the layers of the ground by upward or downward moving an injection stage, although the system is not shown.
The most challenging problems in injection of a grouting material into ground are permeation of the grouting material into a very fine sand layer which has a low coefficient of water permeability and uniform permeation of the grouting material into ground having soil layers different from each other in soil condition.
Generally, coefficient of water permeability, which is represented by k, into a very fine sand layer is such that k=10−3 to 10−4 cm/sec. In order to inject a grouting material into such a soil layer without causing ground breakage, it is necessary in terms of permeation theory that the grouting material be injected under low pressure at a delivery rate of lower than 1 liter to several liters per minute.
In the above-described known injection method, however, since one set of injection pumps is used for each injection pipe, the grouting material is injected inevitably at a delivery rate of 10 to 20 liters per minute because of economical need to minimize a work period and of performance limit of the pump. Accordingly, the injection pressure is high, and thus ground breakage is likely to be caused. This gives rise to ground protuberance, insufficient permeation of the grouting material into a very fine-grained soil layer which leads to insufficient solidification of the soil layer, or the like.
In injection of a grouting material into ground having soil layers different from each other in soil condition such as soil texture, when a soil layer which is subjected to the injection is changed from one to another, it is practically difficult to change delivery rate or to control amount of the grouting material to be injected in response to the change of the soil layer which is subjected to the injection. Accordingly, it is likely that the grouting material spreads throughout one soil layer in a large amount but permeates into another soil layer only in a slight amount. In such injection, there is a problem that continuity between the neighbouring solidified soil layers is not obtained.
Further, a patent application previously filed by the present inventor has been published as Japanese Unexamined Patent Publication No.2000-45259. According to the publication, a plurality of injection pipes are inserted in ground, and a ground improving material is injected into the ground from outlets of the injection pipes in such a manner that the ground improving material is delivered under pressure to the injection pipes and injected from the outlets into the ground by means of a multiple pump (pump plant) comprising a plurality of unit pumps which are concurrently operated by a single driving means.
In the above-described known technique, there is a problem as follows. Since the injection slender pipes are required to extend over a long distance from the multiple pump (pump plant) to the outlets, it is necessary to use a grouting fluid having a low viscosity and a long gelation time. However, if a grouting fluid having a long gelation time once flow out of an intended area to the ground surface or a coarse-grained soil layer in ground, it is inevitable to suspend the injection because no meansures to shorten the gelation time are provided. During the suspension, the grouting fluid disadvantageously gelatinizes in the injection slender pipes.
Further, the unit pumps constituting the multiple pump are concurrently driven by the single driving means. Consequently, all the unit pumps are driven under the same conditions although ground conditions are different at the outlets and thus optimum grouting conditions are different with respect to the outlets. Accordingly, it is impossible to carry out optimum grouting with respect to each of the outlets.